fix(theme): cover the external link icon's paint box with its mask

The icon is a masked background on an inline box whose height follows
font metrics per engine. The fixed centered mask left an unmasked
sliver of the background visible below the arrow on iOS. A full-size
mask can only stretch the arrow slightly, never leak.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/client/theme-default/styles/components/vp-doc.css

@@ -605,8 +605,8 @@
   mask-position: center;
   -webkit-mask-repeat: no-repeat;
   mask-repeat: no-repeat;
-  -webkit-mask-size: 0.6875rem 0.6875rem;
+  -webkit-mask-size: 100% 100%;
-  mask-size: 0.6875rem 0.6875rem;
+  mask-size: 100% 100%;
   /*rtl:raw:transform: scaleX(-1);*/
   vertical-align: middle;
   font-size: 0.5625rem;